1. Field of the Invention
The present invention relates to a surface-treated metal sheet which excels in press-workability, electrical conductivity and corrosion resistance and which is used mainly as a component of household electric appliances, office automation equipment, automobiles and so forth. The invention also is concerned with a method of producing such a metal sheet.
2. Description of the Related Art
Production of household electric appliances, office automation equipment, automobiles and so forth employ press-forming work on various kinds of metal sheets such as non-plated steel sheets, galvanized or galvanealed steel sheets, aluminum (Al) or Al-alloy sheet, for example.
Conventionally, press-forming work has encountered serious problems due to the use of a lubricating oil which is applied to the metal sheet for reducing sliding resistance of the sheet material.
In general, application of lubricating oil is effected by spraying, so that the working environment tends to be adversely affected due to scattering of the lubricating oil.
Degreasing treatment is necessary after press-forming, in order to remove the lubricating oil. The degreasing treatment usually employs a solvent such as 1--1--1 trichloroethane or an alkali detergent. The use of such solvent not only degrades the working environment but also requires a suitable anti-pollution countermeasure which raises the production cost and itself degrades the working environment.
Under these circumstances techniques have been developed to eliminate the necessity of lubricant, as well as degreasing, as disclosed, for example, in Japanese Patent Laid-Open Nos. 60-103185 and 62-73938. These techniques employ so-called self-lubricating steel sheet having a lubricant-containing resin layer containing a variety of lubricants and formed on a chromate layer which overlies a plated steel sheet.
These techniques, however, suffer from a critical problem in that the electrical conductivity inherently possessed by the metal sheet is impaired due to the presence of the resin layer.
More specifically, the resin which generally has a very high volumetric specific resistivity of 10.sup.15 .OMEGA..cm or so produces an inter-layer resistance of 10.sup.10 .OMEGA. or greater on the metal sheet surface even when it is applied as a very thin film of 1 .mu.m or so. Such high electrical resistance seriously impairs the electrical conductivity and grounding characteristics of the product.
In view of this problem, Japanese Patent Laid-Open No. 63-83172 proposed a technique in which conductive particles are dispersed in the resin layer so as to improve electrical conductivity.
According to this technique, however, the electrical conductivity does not recover enough electrical conductivity and grounding characteristics.
For instance, metal sheets for a computer chassis are required to have a high shielding effect against electromagnetic waves, in order to prevent leakage of high-frequency electromagnetic waves to the exterior, as well as generation of noise due to electromagnetic induction. In order to meet this requirement the metal sheet used for such a purpose must have a surface electrical resistivity of 1 .OMEGA. or less.
In order that a sufficiently high level of electrical conductivity is obtained through the technique shown in Japanese Patent Laid-Open No. 63-83172, it is necessary to disperse a large quantity of conductive particles. This not only impedes the work of applying the resin but also impairs the characteristics inherently possessed by the resin. In addition, the high content of the conductive particles in the resin layer tends to impair the corrosion resistance due to galvanic corrosion (corrosion due to contact between metals having different levels of ionization potential) due to enhanced contact between the conductive particles and the underlying metal.
Japanese Patent Laid-Open No. 63-114635 discloses a conductive surface-treated steel sheet having a discontinuous film formed by a resin dispersed on a chromate film.
This steel sheet is disadvantageous in that coating with fine particles cannot easily be conducted with the use of a resin emulsion in electrostatic dispersion coating and in that the particle size of the resin tends to increase after coating due to polymerization of the resin material such as an acrylic emulsion used in the coating process. The increase in the size of the resin particles serving as lubricant undesirably reduces the chance of electrical contact between the metal sheet coated with the lubricant particles and another metal which has to be kept in electrical contact with the coated sheet, thus impairing coating characteristics.
Increase of the lubricant particle size also enhances the tendency of the particles to come off during handling, thus deteriorating workability.
In addition, no specific consideration for improving press workability is given in the art disclosed in Japanese patent Laid-Open No. 63-114635.